Method of synchronising base stations

ABSTRACT

Method of synchronising base stations in a mobile radio telecommunication system, characterised in that a first base station transmits a synchronisation sequence having a first sequence followed by a second sequence, the said first and second sequences being obtained from polyphase complementary sequences and in that at least one second base station effects the correlation of the said synchronisation sequence with a replica of the first sequence and a replica of the second sequence, the correlation results then being added in order to provide synchronisation information.

[0001] The present invention concerns a method of synchronising basestations in a mobile radio telecommunication system. More particularly,the present invention concerns a method of synchronising base stationsfor a telecommunication system of the time division duplex (TDD) type.The said telecommunication system is for example the system for which astandard is at present being drawn up, normally referred to as 3GPPW-CDMA TDD.

[0002]FIG. 1 depicts a radio frame of such a telecommunication system.It consists of fifteen time slots, some of which, for example the slotsIT₀, IT₁, IT₂, IT₅, IT₆ and IT₈, are intended for conveying data (in thebroad sense of the term) in the downlink direction (base station tomobile terminal) whilst others, the slots IT₃, IT₄, IT₇, IT₉, IT₁₀,IT₁₁, IT₁₂, IT₁₃ and IT₁₄, are intended for conveying data in the uplinkdirection (mobile station to base station). During a transmission slot,the data (D) are transmitted in the form of a sequence of symbols. Theslot also includes a midamble (M) comprising pilot symbols enabling thechannel to be estimated, a power control word (TPC) and a guard period(GP′). In such a system, several mobile terminals or base stations cantransmit or receive data in the same time slot. The connections aredifferentiated by code division multiplexing (Code Division MultipleAccess=CDMA). The symbols transmitted by or for the different users arespectrally spread, approximately at a “chip” frequency 1/T_(c) where Tcis the elementary transmission period.

[0003] Because the same frequency can be used both in the uplinkdirection and in the downlink direction, it is essential to ensuresynchronisation of the base stations. This is because, if such were notthe case, a first mobile terminal transmitting at high power in anuplink channel could interfere with a second mobile channel, close tothe first, receiving data over a downlink channel. The synchronisationconstraint between adjacent base stations is around a few microseconds(approximately 5) in the W-CDMA TDD system.

[0004] To effect synchronisation between base stations, several methodshave been proposed in the state of the art. According to a first method,the synchronisation is achieved by virtue of GPS receivers equipping thebase stations. According to a second method, first of all, in an initialphase, for example during the phase of setting up the network or a newbase station, an approximate synchronisation is carried out (of around afew tens of ms, that is to say a few tens of thousands of “chips”). Thisrough initial synchronisation is provided by the network, or moreprecisely by the radio access controller (RNC) controlling severaladjacent base stations (also referred to as “B nodes”). A finesynchronisation is then effected regularly by the radio interfacebetween adjacent base stations. The purpose of this fine synchronisationis notably to correct any difference in the sequencing clocks betweenadjacent base stations. To do this, certain time slots are reserved forthe transmission and reception of a synchronisation signal. A time slotdedicated to synchronisation comprises essentially a synchronisationsequence (Sync) and a guard period (GP). Synchronisation is obtained, ina manner known per se, by correlation of the received sequence with asequence which is a replica of the one transmitted. The correlation iseffected on a time window with a length given by the margin of accuracyof the approximate synchronisation. Thus, when a base station receives asynchronisation sequence and detects a correlation peak in this window,it can synchronise its sequencing with that of the adjoining basestations.

[0005] The synchronisation sequence generally used is lengthy (a fewthousands of “chips”) in order to obtain good accuracy of correlationfor an acceptable power per symbol. The guard period must be greaterthan the propagation time from a base station to an adjacent station soas to avoid, on reception, an encroachment of the synchronisationsequence on an adjacent time slot. The distance between two basestations being greater than the radius of a cell, the guard period (GP)is chosen so as to be greater than the normal guard period (GP′). Theguard period (GP) must also take account of the difference between theframe clocks.

[0006] The synchronisation sequence is chosen so as to have goodautocorrelation properties, namely a very pronounced autocorrelationpeak. Generally the synchronisation sequences used are obtained fromprimitive polynomials on GF(2), a Galois field of cardinal 2. Such asequence has a length L which is an N^(th) power of 2 minus 1, that isto say L=2^(N)−1. This is the case notably for so-called Gold sequencesproposed in the report TSGR1#15(00)0946 entitled “Sequences for the cellsync burst” of the Working Group TSG-RAN of the ETSI for synchronisingadjacent base stations.

[0007] Gold sequences have good periodic autocorrelation properties (thecorrelation of a sequence consisting of the repetition of a Goldsequence with a replica of the sequence of the latter does not havesignificant secondary peaks). On the other hand, these sequencesunfortunately do not have such good aperiodic autocorrelation properties(correlation of an isolated Gold sequence with a replica). What is more,the correlator generally used operates in the time domain in the form ofa conventional adapted FIR filter having a complexity in terms of O(L)which can be very high. In addition, the choice of the lengths of suchsequences is reduced, since they can, as has been seen, take only values2^(N)−1 and a truncation would lead to a substantial loss ofautocorrelation properties.

[0008] One purpose of the present invention is to propose a method ofsynchronising adjacent base stations by virtue of the transmission of acorrelation sequence having very good autocorrelation properties and awide choice of possible lengths, and this for a low degree of complexityof the correlator.

[0009] The present invention is defined by a method of synchronisingbase stations in a mobile radio telecommunication system in which afirst base station transmits a synchronisation sequence having a firstsequence followed by a second sequence, the said first and secondsequences being obtained from polyphase complementary sequences, and atleast one second base station effects the correlation of the saidsynchronisation sequence with a replica of the first sequence and areplica of the second sequence, the correlation results then being addedin order to provide synchronisation information.

[0010] Advantageously, the first and second sequences are Golaycomplementary sequences.

[0011] According to a first embodiment, the said synchronisationsequence comprises guard times around the first and second sequences.

[0012] According to a second embodiment, the said synchronisationsequence comprises a periodic extension of the first sequence followedby a periodic extension of the second sequence.

[0013] According to a third embodiment, the first sequence is generatedby means of a first Golay sequence and a first ancillary sequence bysuccessively multiplying the said first Golay sequence by the bits ofthe first ancillary sequence.

[0014] Likewise, the second sequence can be generated by means of asecond Golay sequence, complementary to the said first Golay sequence,and a second auxiliary sequence by successively multiplying the saidsecond Golay sequence by the bits of the second ancillary sequence.

[0015] Advantageously, the first ancillary sequence and the secondancillary sequence are Golay complementary sequences.

[0016] According to a variant embodiment, the correlation is effected bya trellis filtering.

[0017] The characteristics of the invention mentioned above, as well asothers, will emerge more clearly from a reading of the followingdescription given in relation to the accompanying figures, amongstwhich:

[0018]FIG. 1 depicts schematically a transmission frame of atransmission system of the W-CDMA TDD type;

[0019]FIG. 2A depicts a first embodiment of the invention;

[0020]FIG. 2B depicts a second embodiment of the invention;

[0021]FIG. 2C depicts a third embodiment of the invention;

[0022]FIG. 3 depicts a correlator useful to the third embodiment of theinvention.

[0023] The general idea at the basis of the invention is to use, forsynchronising adjacent base stations, a pair of complementary polyphasecodes and more particularly a pair of Golay complementary codes. In theremainder of the description, mention will be made not of polyphasecodes but of Golay codes. It is clear, however, that the inventionapplies to polyphase codes in general.

[0024] These complementary codes, known as such, have the remarkableproperty that the sum of their aperiodic autocorrelation functions is aDirac function. In other words, if a pair of such complementary codes isdenoted (A,B), this gives a φAA(m)+φBB(m)=δ(m) where m is the timeindex, δ the Kronecker symbol, and φ the aperiodic autocorrelationfunction.

[0025] In addition, as described notably in the article by S. Z.Budisin, entitled “Efficient pulse compressor for Golay complementarysequences”, published in Electronics Letters, Vol. 27, No 3, pages219-220 in January 1991, the correlator can be produced by virtue of atrellis filter having a complexity in terms of 0(logL) rather than interms of O(L) as in a conventional adapted FIR filter. This trellisfilter is also referred to as an EGC filter, standing for EfficientGolay Correlator. An example of an embodiment of an EGC filter is givenin the article by B. M. Popovic entitled “Efficient Golay Correlator”,published in IEEE Electronics Letters, Vol. 35, No 17, January 1999.

[0026] In addition, for a given authorised length, there are severalpossible Golay sequences. This is because, Golay sequences beinggenerated by generator codes, it can be shown that two distinctgenerator codes with the same length generate Golay sequences which arealso distinct and have the same length. These sequences have goodintercorrelation properties (that is to say low intercorrelationvalues), enabling, for example, groups of base stations to use distinctcodes or again to effect a synchronisation of the base stations atdifferent times of their sequencing.

[0027] A first embodiment of the invention is illustrated in FIG. 2A.According to this embodiment, a synchronisation sequence consists of twoGolay complementary sequences A and B multiplexed in time, each sequencebeing preceded and followed by a guard time, as described in the Frenchapplication FR-A-9916851 filed on Dec. 30, 1999 in the name of theapplicant. This sequence is transmitted by a base station and isreceived by an adjacent base station. On reception, the synchronisationsequence is correlated with a replica of the sequence A and a replica ofthe sequence B, and the result of correlation with the sequence A isdelayed so as to be aligned in time with the result of correlation withthe sequence B before they are added, the Dirac peak being obtained whenthe replicas of A and B are aligned with the corresponding sequences.The presence of the guard times GP₁, GP₂ and GP₃ ensures that, at thetime of correlation, the sequences A and B do not overlap thecorresponding complementary replicas, namely B and A respectively, in atime window centred on the time alignment position. Thus secondarycorrelation peaks can result from the intercorrelation between sequencesand complementary replicas are ejected out of this window. Moreprecisely, if GP₂=2.GP₃=2.GP₁=2.GP, the sum of the two correlationresults has an isolated Dirac peak in a window of width 2.GP around thetime alignment position. The correlations are advantageously effected byEGC correlators, as mentioned above.

[0028] A second embodiment of the invention is illustrated in FIG. 2B.According to this embodiment, a synchronisation sequence consists of twoGolay complementary sequences multiplexed in time, each sequence beingpreceded and followed by a periodic extension, as explained in theFrench application entitled “Channel estimation sequence and method ofestimating a transmission channel using such a sequence” filed in thename of the applicant. The periodic extension of a given sequence is atruncation of the periodic sequence obtained by repetition of the saidsequence. To do this, it suffices to concatenate with the sequence to beextended a prefix corresponding to the end and a suffix corresponding tothe start of the said sequence. FIG. 2B indicates schematically theconcatenation of prefixes and suffixes for two Golay complementarysequences A and B. The synchronisation sequence itself consists of twosequences thus extended ext(A) and ext(B). The periodic extensionsproduce the same advantages as the guard times, namely the absence ofsecondary correlation peaks around the Dirac peak in a certain timewindow. More precisely, if the suffixes and prefixes are of identicalsize and equal to E, the sum of the correlation results will have anisolated Dirac peak in a window of width 2.E around the time alignmentposition. This will easily be understood if the case is considered wherethe synchronisation sequence comprises completely periodised sequences Aand B. The correlation with replicas of A and B then produces a seriesof Dirac peaks of period L. A periodic extension of size E amounts totruncating this series by a window of width 2.E around the timealignment peak. The advantage of this embodiment compared with theprevious one is not to cause abrupt variations in signal power betweenthe sequences A and B, at the transmitter amplifier. Such consequentlydegrade the correlation results on reception.

[0029] A third embodiment of the invention is illustrated in FIG. 2C.According to this embodiment, a composite sequence is generated, from aGolay sequence A or B and an ancillary sequence X, according to the modeof constructing the hierarchical sequences. More precisely, the firstbit of the ancillary sequence X is multiplied successively by all thebits of the sequence A, and then the second bit of the second sequenceby all the bits of the sequence A, and so on, and the sequences obtainedare concatenated. Such a composite sequence will be noted below A*X, Abeing the base sequence and X the generator ancillary sequence. TheGolay complementary sequences A and B can thus be multiplied byancillary sequences X, Y, identical or distinct, the latter also beingable themselves to be Golay sequences.

[0030] Let A*X and B*X be composite sequences obtained from a pair A, Bof Golay complementary sequences, of length L, extended by prefixes andsuffixes of size E. A*X and B*X are multiplexed in time and separated byan interval W. The signal received is correlated with the sequence A onthe one hand and with the sequence B on the other hand. The result ofthe first correlation is delayed by (L+2E)+W and is summed with theresult of the second correlation. The sum obtained is a sequence Rhaving a series of Dirac peaks of period L′=L+2E modulated by the valuesx₀, x₁, . . . ,x_(K) where K is the length of the sequence X, each peakbeing surrounded by a window of width 2.E containing only zeros. Thesequence R is then subjected to a filtering by means of a linearresponse filter:

H(z)=x ₀ +x ₁ ·z ^(−L′) +. . . +x _(K) ·z ^(−K·L′)

[0031] The filtered sequence R includes a Dirac peak of height 2·K·L inthe middle of a zero window of width 2.E which makes it possible todetect it easily. In addition, the total sequence consisting of thesequences A*X and B*X multiplexed in time is of total length2·(L+2·E)·K+W, which offers a wide choice of lengths of permittedsequences.

[0032] According to another variant embodiment, four composite sequencesA*X, A*Y, B*X, B*Y are generated, where A, B form a first pair of Golaycomplementary sequences, extended or not, and X, Y form a second pair ofGolay complementary sequences serving as generator ancillary sequences.

[0033] The composite sequences are multiplexed in time and separated byintervals which will be assumed to be equal and of width W. Thesequences A and B are of length L′=L+2.E where L is the length of thebasic sequence and E the size of the extension, the sequences X, Y beingof length K. The total sequence length is therefore 4(L+2E)K+3W, whichoffers a wide choice of permitted sequence lengths.

[0034] The present variant takes advantage of the fact that there are L′pairs of complementary sequences (X,Y) in the form of sub-sequencesS_(m) and S′_(m) with S_(m)(n)=(A*X)_(n·L′+m) andS′_(m)(n)=(B*X)_(n·L′+m), m=0, . . . ,L′−1 obtained by decimation of theinitial total sequence. Instead of effecting a correlation with an EGCcorrelator, a “hierarchical” correlator is used, the first stage of theEGC function correlator modified as depicted in FIG. 3.

[0035] It will be assumed that the pair of sequences X and Y has beengenerated conventionally by an elementary sequence s₀, . . . ,s_(k−1),where K=2^(k)−1, and delays D′₀,D′₁, . . . ,D′_(k−1) with D′_(i)=2^(Pi)where (P₀, P₁, . . . , P_(k−1)) is a permutation on the set (0,1, . . ., k−1), recursively as follows:

X₀(i)=δ(i); Y₀(i)=δ(i);

X _(n)(i)=X _(n−1)(i)+S_(n−1·) X _(n−1)(i−D′ _(i)); Y _(n)(i)=Y_(n−1)(i)−S _(n−1)·Y_(n−1)(i−D′ _(i));

[0036] Likewise, it will be assumed that the pair of sequences A, B wasgenerated by the elementary sequence t₀, . . . , t₁₋₁, where L=2¹−1, anddelays D₀, D₁, . . . , D_(k−1) with D_(i)=2^(Pi) where (P₀, P₁, . . . ,P₁₋₁) is a permutation on the set (0,1, . . . , 1-1).

[0037] The first correlation stage effects a correlation with the pairof sequences X, Y, but differs from a conventional EGC correlator inthat the delays have been multiplied by a factor L′ in order to takeaccount of the scattering in the samples. The two correlation resultsare added after time alignment by a delay D_(XY), the delay D_(XY)separating the sequences A*X and A*Y, on the one hand, the sequences B*Xand B*Y, on the other hand. The second stage of the correlator effectsthe correlation with the pair of sequences A, B and is conventional perse. The correlation results are aligned in time by a delay D_(AB) andadded, the delay D_(AB) corresponding to the difference in time betweenthe sequences A*X and B*X on the one hand and the sequences A*Y and B*Yon the other hand.

[0038] The correlator thus formed first of all effects a roughcorrelation with a step L′ and then a fine correlation to the samplingstep. Its complexity is low since the number of operations performed isin O(log(K)+log(L)).

[0039] Although the example described above has only two sequence levelsand two correlation levels, the invention can be extended in animmediate manner to any number of levels of sequences and correspondingstages of the hierarchical correlator.

1. Method of synchronising base stations in a mobile radiotelecommunication system, characterised in that a first base stationtransmits a synchronisation sequence having a first sequence followed bya second sequence, the said first and second sequences being obtainedfrom polyphase complementary sequences and in that at least one secondbase station effects the correlation of the said synchronisationsequence with a replica of the first sequence and a replica of thesecond sequence, the correlation results then being added in order toprovide synchronisation information.
 2. Synchrsonisation methodaccording to claim 1, characterised in, that the first and secondsequences are Golay complementary sequences.
 3. Synchronisation methodaccording to claim 1 or 2, characterised in that the saidsynchronisation sequence comprises guard times around the first andsecond sequences.
 4. Synchronisation method according to claim 1 or 2,characterised in that the said synchronisation sequence comprises aperiodic extension of the first sequence followed by a periodicextension of the second sequence.
 5. Synchronisation method according toclaim 2, characterised in that the first sequence is generated by meansof a first Golay sequence and a first ancillary sequence by successivelymultiplying the said first Golay sequence by the bits of the firstancillary sequence.
 6. Synchronisation method according to claim 5,characterised in that the second sequence is generated by means of asecond Golay sequence, complementary to the said first Golay sequence,and a second ancillary sequence by successively multiplying the saidsecond Golay sequence by the bits of the second ancillary sequence. 7.Synchronisation method according to claim 6, characterised in that thefirst ancillary sequence and the second ancillary sequence are Golaycomplementary sequences.
 8. Synchronisation method according to one ofthe preceding claims, characterised in that the correlation is effectedby a trellis filtering.